1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to a disk drive for detecting a polarity error of a magnetoresistive head by detecting a sync mark.
2. Description of the Prior Art
FIG. 1 shows a prior art format of a disk 2 employed in a disk drive of a computer system. The disk 2 comprises a plurality of radially-spaced, concentric tracks 4 which are partitioned into a plurality of data sectors. The disk 2 further comprises a plurality of embedded servo sectors 6 for use in servoing a head over the desired track during write and read operations. A sector (data or servo) typically comprises a preamble field 8 for synchronizing timing recovery and gain control within a read channel, and a sync mark 10 for symbol synchronizing to a data field 12. Data stored in an embedded servo sector 6 may include a track address which provides coarse head positioning information to a servo control system. An embedded servo sector 6 also typically comprises a plurality of servo bursts 14 recorded at precise offsets from a track""s centerline to provide fine head positioning information to the servo control system.
Magnetoresistive (MR) heads (including the Giant Magnetoresistive (GMR) and spin valve MR heads) are typically employed in disk drives due to the increase in sensitivity as compared to a conventional inductive head. An MR head comprises an inductive write element (coil) and a MR read element which has a resistance inversely proportional to the strength of the magnetic field created by the magnetic flux transitions recorded on the disk. The MR read element typically comprises a bias layer that is magnetized according to a particular orientation or polarity. It is possible for the bias layer to deviate from its preferred polarity due, for example, to the magnetic field induced by the write element or the magnetic field emanating from the disk, particularly in the presence of a thermal asperity.
When the bias layer of the MR head deviates from its preferred polarity, the sensitivity of the MR read element is greatly reduced. It is therefore desirable to detect when the bias layer of an MR head has deviated from the preferred polarity, and to realign the polarity of the bias layer toward its preferred orientation.
The present invention may be regarded as a disk drive comprising a disk having a plurality of data tracks, each data track comprising a plurality of sectors. Each sector comprises a preamble field for storing a predetermined periodic data sequence, a sync field for storing a predetermined sync mark pattern, and a data field for storing data. A magnetoresistive (MR) head comprising a MR read element having a bias layer magnetizable toward a preferred polarity is actuated radially over the disk for generating a read signal. A sync mark detector detects the sync mark pattern in the read signal, wherein when the sync mark pattern is detected the sync mark detector generates a sync mark detect signal and a polarity signal. The polarity signal is indicative of a polarity of the bias layer. When the polarity signal indicates the polarity of the bias layer has deviated from the preferred polarity, the polarity of the bias layer is modified.
In one embodiment the sync mark detector comprises a matched filter and a dual polarity correlator. In one embodiment the polarity of the bias layer is modified by increasing a bias current applied to the bias layer for a predetermined interval.
The present invention may also be regarded as a method of compensating for a polarity error of a bias layer in a magnetoresistive (MR) head deployed in a disk drive. The disk drive comprises a disk having a plurality of data tracks, wherein each data track comprises a plurality of sectors. Each sector comprises a preamble field for storing a predetermined periodic data sequence, a sync field for storing a predetermined sync mark pattern, and a data field for storing the data. The method comprises the steps of detecting the sync mark pattern in the read signal generated by the MR head, and when the sync mark pattern is detected, generating a sync mark detect signal and a polarity signal. The polarity signal is indicative of a polarity of the bias layer. When the polarity signal indicates the polarity of the bias layer has deviated from the preferred polarity, the polarity of the bias layer is modified.